Negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery comprising the same

ABSTRACT

The negative active material for a rechargeable lithium battery of the present invention includes a carbonaceous material and a silicon-based compound represented by Formula 1:
 
Si (1-y) M y O 1+x    (1)
 
where 0&lt;y&lt;1, −0.5≦x≦0.5, and M is selected from the group consisting of Mg, Ca, and mixtures thereof.

CROSS REFERENCES TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication Nos. 10-2004-0012773, and 10-2004-0012774, both filed onFeb. 25, 2004, and both of which are hereby incorporated by referencesfor all purposes as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to a negative active material for arechargeable lithium battery, a method of preparing the same, and arechargeable lithium battery comprising the same, and particularly, to anegative active material for a rechargeable lithium battery having goodcycle-life characteristics and good charge and discharge characteristicat a high rate, a method of preparing the same, and a rechargeablelithium battery comprising the same.

BACKGROUND OF THE INVENTION

The use of portable electronic instruments is increasing as electronicequipment gets smaller and lighter due to developments in the high-techelectronic industries. Studies on rechargeable lithium batteries areactively being pursued in accordance with an increased need forbatteries having high energy density for use as power sources in theseportable electronic instruments. Even though graphite is suggested forthe negative active material as it has a theoretical capacity of 372mAh/g, a material with a higher capacity than graphite is desired.

Silicon or silicone compounds have been proposed as a substitute forgraphite. The silicon or silicone compounds are known to be alloyed withlithium and have a higher electric capacity than graphite.

Recently, the following have been proposed for substitution with theconventional graphite material: (a) a simple mixture of a graphite and asilicone compound powder, (b) a material in which a pulverized siliconecompound is chemically fixed on the surface of graphite by a silanecoupling agent, and (c) a material in which an element such as Si isbound with or coated on the graphite-based carbonaceous material.

However, regarding (a) a simple mixture of graphite and siliconecompound powder, the graphite is not completely contacted with thesilicone compound so that the silicone compound is released from thegraphite when the graphite is expanded or contracted upon repeating thecharge and discharge cycles. Therefore, as the silicone compound has lowelectro-conductivity, the silicone compound is insufficiently utilizedfor a negative active material and the cycle characteristics of therechargeable lithium battery deteriorate.

Regarding (b) a material in which a pulverized silicone compound ischemically fixed on the surface of graphite by a silane coupling agent,although the resulting material works as a negative active material, atthe early charge and discharge cycles, problems arise in that thesilicone compound expands when it is alloyed with the lithium uponrepeating the charge and discharge cycles. Therefore, the linkage of thesilane coupling agent is broken to release the silicone compound fromthe graphite so that the silicone compound is insufficiently utilized asa negative active material. As a result, the cycle characteristics ofthe rechargeable lithium battery deteriorate. Further, the silanecoupling agent may not be uniformly treated upon preparing the negativeelectrode material so that it is difficult to provide a negativeelectrode material having a constant quality.

Regarding (c) a material in which an element such as Si is bound with orcoated on the graphite-based carbonaceous material, such a material hassimilar problems as those of (b) a material in which the pulverizedsilicone compound is chemically fixed on the surface of graphite by asilane coupling agent. That is, upon progressing through charge anddischarge cycles, the linkage of the amorphous carbonaceous material canbe broken by the expansion of the material alloyed with the lithium. Thematerial is thereby released from the graphite carbonaceous material andis insufficiently utilized as a negative active material. As a result,the cycle characteristics deteriorate.

SUMMARY OF THE INVENTION

In embodiments of the present invention, a negative active material isprovided for a rechargeable lithium battery having improved cycle-lifecharacteristics and charge and discharge characteristic at a high rate,as well as a method of preparing the same.

In another embodiment of the present invention, a rechargeable lithiumbattery is provided comprising the negative active material.

In one embodiment of the present invention, a negative active materialis provided for a rechargeable lithium battery which comprises acarbonaceous material and a silicone-based compound represented by thefollowing Formula 1:Si_((1−y))M_(y)O_(1+x)  (1)where 0<y<1, −0.5≦x≦0.5 (mol fraction), and M is selected from the groupconsisting of Mg, Ca, and mixtures thereof.

A method of preparing the negative active material for a rechargeablelithium battery is also provided, the method comprising the steps of:mixing SiO₂, Si, and an M-included compound (where M is selected fromthe group consisting of Mg, Ca, and mixtures thereof) to provide amixture; heating the mixture to provide a silicone-based compoundrepresented by the following Formula 1:Si_((1−y))M_(y)O_(1+x)  (1)where 0<y<1, −0.5≦x≦0.5 (mol fraction), and M is selected from the groupconsisting of Mg, Ca, and mixtures thereof; quenching the heatedsilicone-based compound; and mixing the quenched silicone-based compoundand a carbonaceous material.

In another embodiment of the present invention, a rechargeable lithiumbattery is provided comprising a negative electrode comprising thenegative active material described above; a positive electrodecomprising a positive active material capable of reversiblyintercalating/deintercalating the lithium; and an electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawing, wherein:

FIG. 1 is a perspective view showing one embodiment of a rechargeablelithium battery according to the present invention.

DETAILED DESCRIPTION

In the following detailed description, certain preferred embodiments ofthe invention have been shown and described, simply by way ofillustration of the best mode contemplated by the inventors of carryingout the invention. As will be realized, the invention is capable ofmodification in various obvious respects, all without departing from thescope of the invention. Accordingly, the drawings and description are tobe regarded as illustrative in nature, and not restrictive.

The negative active material according to an embodiment of the presentinvention comprises a carbonaceous material and a silicon-based compoundrepresented by the following Formula 1:Si_((1−y))M_(y)O_(1+x)  (1)where 0<y<1, −0.5≦x≦0.5 (mol fraction), and M is selected from the groupconsisting of Mg, Ca, or mixtures thereof.

In Formula 1, y is preferably between 0 and 1, and more preferablybetween 0 and 0.5. Furthermore, in Formula 1, x is preferably between−0.5 and 0.5, and more preferably between −0.2 and 0.2. When x is morethan 0.5, the irreversible capacity may increase due to reactions withthe lithium causing the early-stage efficiency to deteriorate.

Generally, the silicon oxide represented by Formula 2:SiO_(1+z)  (2)where 0≦z≦1, is known to be unsuitable for the negative active materialfor a rechargeable lithium battery, because of its high irreversiblecapacity, short cycle life, and low charge and discharge efficiency athigh rate. This is because it has a stable structure upon repeating thecharge and discharge, and a low diffusion speed of Li atoms.

As the present invention employs the silicon-based compound representedby Formula 1 for a negative active material by introducing Mg, Ca, or amixture thereof into a silicon oxide compound of Formula 2, it ispossible to increase the amorphorization degree of the negative activematerial, to prevent the Si-metal aggregate from growing when thenegative active material is reduced, and to improve the diffusion speedof Li atoms.

According to a first embodiment of the present invention, a negativeactive material of a complex of a silicon-based compound and acarbonaceous material includes a core of a silicon-based compoundrepresented by Formula 1 which is obtained by introducing Mg, Ca, or amixture thereof into a silicon oxide compound of Formula 2, with acarbonaceous material coated on the surface of the core.

According to a second embodiment of the present invention, a negativeactive material includes a mixture of a carbonaceous material and asilicon-based compound represented by Formula 1 which is obtained byintroducing Mg, Ca, or a mixture thereof into a silicon oxide compoundof Formula 2.

The amorphorization degree of the negative active material according tothe present invention is 70% or more, and preferably between 70 and 99%.Furthermore, the diffusion speed of Li atoms of the negative activematerial is 10⁻⁸ cm²/sec or more, and preferably between 10⁻⁸ and 10⁻⁶cm²/sec determined according to GITT (Galvanostatic IntermittentTitration Technique). The amorphorization degree is defined by thefollowing Calculation Formula: Amorphorization degree (%)=((Main XRDpeak intensity of silicon-based compound after carrying out quenchingtreatment)/((Main XRD peak intensity of silicon-based compound beforecarrying out quenching treatment))×100.

The carbonaceous material coated or mixed with the silicon-basedcompound may include crystalline carbon or amorphous carbon. Thecrystalline carbon may include a sheet-, a spherical-, or a fiber-shapednatural graphite or artificial graphite. The amorphous carbon may be anyone of graphitizable carbon (soft carbon, sintered carbon at a lowtemperature), and non-graphitizable carbon (hard carbon). The softcarbon can be obtained by heating a coal pitch, a petroleum pitch, atar, or a heavy oil having a low molecular weight at 1000° C. The hardcarbon can be obtained by heating a phenol resin, a naphthalene resin, apolyvinyl alcohol resin, a urethane resin, a polyimide resin, a furanresin, a cellulose resin, an epoxy resin, or a polystyrene resin at1000° C. Further, it can be obtained by optional non-deliquescence of amesophase pitch, raw coke, and a carbonaceous material in which thepetroleum, the coal-based carbonaceous material, or the resin-basedcarbon is heated at between 300 and 600° C., and heating the same atbetween 600 and 1500° C.

The silicon-based compound and the carbonaceous material are preferablymixed in a weight ratio of between 95:5 and 50:50, more preferablybetween 95:5 and 70:30, and still more preferably between 80:20 and60:40.

Hereinafter, a method of preparing a silicon-based compound composingthe negative active material according to one embodiment of the presentinvention is described. An M-included compound (wherein M is Mg, Ca, ora mixture thereof) is added to a mixture of SiO₂ and Si, and the mixtureis heated. SiO₂ and Si are preferably mixed in a molar ratio of between1:1 and 1:3.

The M-included compound (where M is Mg, Ca, or a mixture thereof) ispreferably a glass network precursor. Suitable Mg-included compoundsinclude MgO, and suitable Ca-included compounds include CaO. Thesecompounds are preferably added at 20 parts to 50 parts by weight basedon 100 parts by weight of a mixture of SiO₂ and Si.

The heating temperature is preferably between 600 and 1000° C., and morepreferably between 800 and 1000° C. When the heating temperature is lessthan 600° C., it is difficult to provide a uniform silicon-basedcompound due to the deteriorated heat diffusion. Further, when it ismore than 1000° C., an undesirable decomposition reaction of Si mayoccur. The heating process is preferably carried out under an inertatmosphere or a vacuum atmosphere. According to the invention, Mg, Ca,or both are introduced onto the silicon-based compound to improve thediffusion speed of the lithium and the amorphorization degree of thenegative active material.

After the heating process, the compound is quenched to form glass. Thequenching process may include, but is not limited to, water-cooling ormelt-spinning methods. In the melt-spinning method, the melted materialis sprayed via a fine nozzle by gas having a specific pressure to ametal roll (typically, a Cu-roll) rotating at a high speed and having asurface temperature at room temperature or less. The quenching speed ispreferably between 10² and 10⁷ K/sec.

The silicon-based compound including Mg, Ca, or both is provided by theheating and the quenching processes. Then the resulting silicon-basedcompound is coated or mixed with a carbonaceous material to provide anegative active material. These two methods are referred to as the“coating” or “mixing” methods, respectively.

The silicon-based compound and the carbonaceous material are preferablyused in a weight ratio of between 95:5 and 50:50, more preferablybetween 95:5 and 70:30, and still more preferably between 80:20 and60:40.

The carbonaceous material may be crystalline carbon or amorphous carbon.

For crystalline carbon, the negative active material made by the coatingmethod can be obtained by mixing the core material and crystallinecarbon in either a solid phase or a liquid phase, and subsequentlycoating the crystalline carbon on the core. For the mixing method, thenegative active material can be obtained by mixing the core material andcrystalline carbon in either a solid phase or a liquid phase.

For a solid-phase mixing method, the mixing step may be performed bymechanically mixing the core material with crystalline carbon.Mechanical mixing may be accomplished by kneading, or using a mixerhaving a mixing blade with a modified wing structure compared aconventional mixing blade so as to provide sufficient shear stress tothe mixture. Alternatively, a mechano-chemical mixing technique may beused where shear strength is applied to particles in order to causefusion between particle surfaces.

For a liquid-phase mixing method, the mixing step may be performedeither by mechanically mixing the core material with crystalline carbon,or by spray-drying, spray-pyrolysis, or freeze-drying. Possible solventsinclude water, organic solvents, or mixtures thereof. Possible organicsolvents include ethanol, isopropyl alcohol, toluene, benzene, hexane,tetrahydrofuran, and the like.

For amorphous carbon, the carbon material can be formed by heat-treatingthe mixture of a core material coated with the carbon materialprecursor. The coating process may be performed using a dry or wetmethod. Additionally a deposition method such as chemical vapordeposition (CVD) may be performed using a carbon-included gas such asmethane, ethane or propane. For the carbon material precursor used forcoating the carbonaceous material over the core, at least one materialselected from the group consisting of various resins such as phenolicresin, naphthalene resin, polyvinylalcohol resin, urethane resin,polyimide resin, furan resin, cellulose resin, epoxy resin, andpolystyrene resin; coal-based pitch; petroleum-based pitch; tar; orheavy oil with a low molecular weight may be used. However, it isunderstood that the carbon material precursor in the present inventionis not limited thereto.

A rechargeable lithium battery according to one embodiment of thepresent invention comprises a negative electrode composed of thenegative active material described above. The negative electrode isprepared by mixing the negative active material with a conductive agentand a binder to provide a negative electrode mass, and coating the sameon a current collector of copper.

The conductive agent may include, but is not limited to, nickel powder,cobalt oxide, titanium oxide, or carbon. The carbon for the conductiveagent may include ketchen black, acetylene black, furnace black,graphite, carbon fiber, or fullerene, and is preferably graphite.

FIG. 1 shows a rechargeable lithium battery 1 according to an embodimentof the present invention. The rechargeable lithium battery 1 includes anegative electrode 2, a positive electrode 3, and a separator 4interposed between the positive electrode 3 and the negative electrode2, all of which are placed in a cell housing 5 filled with electrolyteand sealed with a sealing member 6. Even though the rechargeable lithiumbattery shown in FIG. 1 is formed in a cylindrical shape, it may beformed into various shapes such as a prismatic, a coin, or a sheetshape.

The positive electrode may be constructed of a positive electrode masscomprising a positive active material, a conductive agent, and a binder.Suitable positive active materials include compounds capable ofreversibly intercalating/deintercalating lithium ions such as LiMn₂O4,LiCoO₂, LiNiO₂, LiFeO₂, V₂O₅, TiS, or MoS. Suitable materials for theseparator include olefin-based porous films such as polyethylene orpolypropylene.

Suitable electrolytes include lithium salts dissolved in a solvent.Suitable lithium salts include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄,LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiSbF₆, LiAlO₄, LiAlCl₄,LiN(C_(a)F_(2a+1)SO₂)(C_(b)F_(2b+1)SO) (wherein a and b are naturalnumbers), LiCl, LiI, and mixtures thereof. Suitable solvents includeethylene carbonate, propylene carbonate, butylene carbonate,benzonitrile, acetonitrile, tetrahydrofurane, 2-methyl tetrahydrofurane,γ-butyrolactone, dioxolane, 4-methyldioxolane, N,N-dimethylformamide,dimethylacetoamide, dimethylsulfoxide, dioxane, 1,2-dimethoxyethane,sulforane, dichloroethane, chlorobenzene, nitrobenzene, dimethylcarbonate, methylethyl carbonate, diethyl carbonate, methylpropylcarbonate, methylisopropyl carbonate, ethylbutyl carbonate, dipropylcarbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol,dimethyl ether, and mixtures thereof.

Furthermore, instead of a liquid electrolyte as set forth above, a solidpolymer electrolyte may be used. It is preferred that if a polymerelectrolyte is used, it should employ a polymer having ion-conductivityto lithium ions, and examples include polyethylene oxide, polypropyleneoxide, and polyethyleneimine. The electrolyte may also be in a gel statesuch that the solvent and the solute are added to the polymer.

The following examples further illustrate the present invention indetail, but are not to be construed to limit the scope thereof.

EXAMPLE 1

SiO₂ and Si were mixed in a molar ratio of 1:1 to provide a mixture. Tothe mixture, 9 parts by weight of MgO based on 100 parts by weight ofthe mixture were added, mixed, then heated at 900° C. under a vacuum andquenched at a rate of 10⁷ K/sec by a melt spinning technique to provideSi_(0.9)Mg_(0.1)O. The resulting Si_(0.9)Mg_(0.1)O was mixed withgraphite at a weight ratio of 1:1 to provide a negative active material.

EXAMPLE 2

SiO₂ and Si were mixed in a molar ratio of 1:1 to provide a mixture. Tothe mixture, 12 parts by weight of CaO based on 100 parts by weight ofthe mixture were added, mixed, then heated at 900° C. under a vacuum andquenched at a rate of 10⁷ K/sec by a melt spinning technique to provideSi_(0.9)Ca_(0.1)O. The resulting Si_(0.9)Ca_(0.1)O was mixed withgraphite at a weight ratio of 1:1 to provide a negative active material.

COMPARATIVE EXAMPLE 1

SiO₂ and Si were mixed at a molar ratio of 1:1 and heated at 900° C.under a vacuum and quenched at a rate of 10⁷ K/sec to provide SiO. Theresulting SiO and graphite were mixed at a weight ratio of 1:1 toprovide a negative active material.

EXAMPLE 3

SiO₂ and Si were mixed in a molar ratio of 1:1 to provide a mixture. Tothe mixture, 9 parts by weight of MgO based on 100 parts by weight ofthe mixture were added, mixed, then heated at 900° C. under a vacuum andquenched at a rate of 10⁷ K/sec to provide Si_(0.9)Mg_(0.1)O. Theresulting Si_(0.9)Mg_(0.1)O was coated with 30% by weight of amorphouscarbon material using chemical vapor deposition (CVD) to provide anegative active material.

COMPARATIVE EXAMPLE 2

A Si powder having a particle size of 5 μm was coated with 30% by weightof amorphous carbon material using chemical vapor deposition (CVD) toprovide a Si-complex negative active material coated with carbonmaterial.

COMPARATIVE EXAMPLE 3

SiO₂ and Si were mixed in a molar ratio of 1:1, then heated at 900° C.under a vacuum and quenched at a rate of 10⁷ K/sec to provide SiO. Theresulting SiO was coated with 30% by weight of amorphous carbon materialusing chemical vapor deposition (CVD) to provide a negative activematerial.

Fabricating a Test Cell for Measuring the Charge and Discharge

The negative active materials according to Examples 1 to 3 andComparative Examples 1 to 3 were mixed with polyfluorovinylidene in aratio of 90:10 in N-methylpyrrolidone to provide a negative electrodeslurry solution. The slurry solution was applied with a doctor blade toa copper foil having a thickness of 18 μm, and heated under a vacuumatmosphere at 100° C. for 24 hours to evaporate the N-pyrrolidone. Anegative active mass having a thickness of 120 μm was thereby depositedon the cupper foil, which was then cut to form a circle with a diameterof 13 mm to thereby provide a negative electrode.

In addition to the negative electrode, lithium metal foil was punched ina circle shape having the same diameter as the negative electrode toprovide a counter electrode, and a separator composed of a porouspolypropylene film was inserted between the negative electrode and thecounter electrode to provide a coin-type test cell. For the electrolyte,1 mol/L of LiPF₆ solution was dissolved in a mixed solvent of propylenecarbonate (PC), diethyl carbonate (DEC), and ethylene carbonate (EC) ata volume ratio of PC: DEC: EC of 1:1:1.

The charge and discharge tests were performed for the negative activematerials according to Examples 1 and 2 and Comparative Example 1 underthe condition of a 0.2 C charge and discharge rate, a cut-off chargevoltage of 0 V (Li/Li⁺), and cut-off discharge voltage of 2.0 V(Li/Li⁺), and the results are summarized in Table 1.

In the following Table 1, initial irreversible capacity was measuredusing galvanostatic charge/discharge tester. The capacity at 2C wasmeasured using galvanostatic charge/discharge test, and cycle liferetention ratio was measured after 100 cycle charging and discharging at0.2C. The diffusion rate of the lithium (Li) was measured according toGITT (Galvanostatic Intermittent Titration Technique).

TABLE 1 Cycle Life Retention Ratio (%) of Ratio (%) after Early StageCapacity at 2C 100 Charge and Diffusion Irreversible Relative toDischarge Rate of Li Capacity (%) that at 0.2C Cycles (cm²/sec) Example1 90 80 70 2.1 × 10⁻⁸ Example 2 90 80 70 2.2 × 10⁻⁸ Comparative 65 50 30 1 × 10⁻¹⁰ Example 1

As shown in Table 1, the rechargeable lithium battery comprisingnegative active materials according to Examples 1 and 2 have improvedearly stage irreversible capacity at 25% higher, the capacity at 2Crelative to 0.2C at 30% higher, and the cycle-life retention rate at 40%higher, than the rechargeable lithium battery comprising the negativeactive material according to Comparative Example 1. Furthermore, thelithium was very rapidly diffused.

With respect to the negative active materials of Example 3 andComparative Examples 2 and 3, discharge capacity, initial efficiency,and cycle life and amorphorization degree of the active materials weremeasured. The amorphorization degree is defined by the followingCalculation Formula 1: Amorphorization degree (%)=((Main XRD peakintensity of silicon-based compound after carrying out quenchingtreatment)/((Main XRD peak intensity of silicon-based compound beforecarrying out quenching treatment))×100.

The results are shown in Table 2.

TABLE 2 Cycle Life After 100 Initial Charge and Amorpho- DischargeEfficiency Discharge rization Capacity (mA/g) (%) Cycles (%) DegreeExample 3 700 88 >90 80 Comparative 1200 90 <40 0 Example 2 Comparative850 78 <70 50 Example 3

As described in Table 2, the negative active material of Example 3 hashigher amorphorization degree and improved cycle life characteristicscompared to the negative active materials of Comparative Examples 2 and3.

As described above, the negative active material for a rechargeablelithium battery according to the present invention is capable ofincreasing the amorphorization degree of the negative active material byintroducing Mg, Ca, or a mixture thereof into a silicon-based mixtureand accelerating the diffusion rate of Li atoms. Thereby, the cycle-lifecharacteristics and the charge and discharge characteristics at highrate of the rechargeable lithium battery are improved.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. A negative active material for a lithium secondary battery,comprising: a carbonaceous material and a silicon-based compoundrepresented by the following formula:Si_((1−y))M_(y)O_(1+x) where 0<y<1, −0.5≦x≦0.5 and M is selected fromthe group consisting of Mg, Ca, and mixtures thereof.
 2. The negativeactive material for a lithium secondary battery according to claim 1,wherein the negative active material has an amorphorization degree of70% or more, and a diffusion rate of Li measured by GITT (GalvanostaticIntermittent Titration Technique) of 10⁻⁸ cm²/sec or more.
 3. Thenegative active material for a lithium secondary battery according toclaim 1, wherein the negative active material has an amorphorizationdegree of between 70% and 99%, and a diffusion rate of Li measured byGITT (Galvanostatic Intermittent Titration Technique) of between 10⁻⁸and 10⁻⁶ cm²/sec.
 4. The negative active material for a lithiumsecondary battery according to claim 1, wherein y is between 0 and 0.5,and x is between −0.2 and 0.2.
 5. The negative active material for alithium secondary battery according to claim 1, wherein the carbonaceousmaterial is selected from crystalline carbon and amorphous carbon. 6.The negative active material for a lithium secondary battery accordingto claim 1, wherein the silicon-based compound and the carbonaceousmaterial are mixed in a weight ratio of between 95:5 and 50:50.
 7. Thenegative active material for a lithium secondary battery according toclaim 1, wherein the carbonaceous material is coated on the surface ofthe silicon-based compound.
 8. The negative active material for alithium secondary battery according to claim 1, wherein the negativeactive material is a mixture of the silicon-based compound and thecarbonaceous material.
 9. A lithium secondary battery comprising: anegative electrode comprising the negative active material according toclaim 1; a positive electrode comprising a positive active materialcapable of reversibly intercalating/deintercalating the lithium; and anelectrolyte.